Inverse vertical seismic profiling using a measurement while drilling tool as a seismic source

ABSTRACT

A reverse VSP system is provided and uses a frequency-controllable modulator of an MWD tool as a downhole seismic source. The modulator has a rotor and stator arranged to provide substantially sinusoidal outputs, and is controlled to provide a &#34;Vibroseis&#34;-type frequency sweep (preferably from 1 Hz to 50 Hz). With the bit of the MWD tool in contact with the bottom of the borehole, the force generated by the frequency-controllable modulator will generate measurable axial compressional (P-) waves. These waves which move through the formation are detected by an array of geophones on the formation surface, and a reference signal is obtained by a kelly-mounted accelerometer or strain gauge. The reference signal and signals obtained by the geophones are processed using known signal processing techniques. The disclosed downhole frequency-controllable seismic source provides many advantages including: good resolution due to the ability to output a sinusoidal wave at desired frequencies and the fact that the location of the source signal is well-defined (at the bit), the ability to image formations ahead of the bit, and the ability to conduct reverse VSP processing without adding downhole hardware equipment and without requiring trips out of the well.

This is a division of application Ser. No. 08/614,288, filed Mar. 12,1996, now U.S. Pat. No. 5,901,113.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the profiling of geological formations.More particularly, the present invention relates to the use of adrilling tool as a frequency-controlled seismic source for the inverseor reverse vertical seismic profiling or imaging of a formation.

2. Description of the Related Art

Conventional reflection seismology utilizes surface sources and surfacereceivers to detect reflections from subsurface acoustic impedancecontrasts; i.e., subsurface strata. The image and information obtainedutilizing conventional reflection seismology is considered relativelypoor due to long travel paths between the sources, reflectors, andreceivers.

A technique known as vertical seismic profiling (VSP) has been used toprovide more accurate information regarding the formation in thevicinity of a borehole. In VSP, seismic sources are located at thesurface, and sensors (geophones) are suspended in the borehole with awireline. When the sources are fired, the received data is recorded andprocessed for several borehole depths. VSP has the drawbacks, however,that each surface source is costly to apply, adds to acquisition timeand rig inactivity, and generates multiple ghost images due to energytrapped in the surface layer. In addition, in drilling applicationswhere the geophones would have to be added to an MWD (measurement whiledrilling) tool string, unless the downhole geophones could be decoupledfrom the drill string, they would only be capable of recording therelatively large first arrival event. Furthermore, downhole geophonesare sensitive to tube wave events traveling up and down the borehole.Thus, to date, vertical seismic profiling has not been successful in MWDapplications.

In order to overcome some of the problems relating to explosive sourcesused in conjunction with VSP, "Vibroseis" (a trademark of ConocoIncorporated) sources have been utilized. The "Vibroseis" source(described in U.S. Pat. No. 2,688,124) comprises vibrators which arecarried on trucks which are coupled to the formation surface and whichgenerate a frequency sweep from 10-90 Hz over a period of time. Thetrucks move to pre-designated source locations, lower their heavybaseplates, and start vibrating through the use of a hydraulic system.However, the "Vibroseis" source technique still suffers from severaldrawbacks. These include difficulty in correctly locating the trucks andin establishing good ground contact.

Long ago it was recognized that it might be possible to switch thelocation of the seismic source and the detectors; i.e., provide an"inverse" or "reverse" VSP (hereinafter referred to as "reverse VSP" or"RVSP"). A downhole source overcomes the logistical limitations inherentin using multiple surface sources, and permits simultaneous dataacquisition at the surface with little cost.

Various different downhole source schemes have been proposed over theyears. Some of the downhole source schemes utilize the drill bit as thesource, while others use sources other than the drill bit. Of theformer, U.S. Pat. No. 2,062,151 to Weatherby suggests using the drillbit as an impulse generator of seismic waves, while U.S. Pat. No.4,363,112 and U.S. Pat. No. 4,365,322 to Widrow suggest using thecontinuous natural random vibrations of the drill bit to launch seismicwaves. Techniques such as disclosed in U.S. Pat. No. 5,050,130 to Rectoret al., can be useful in processing the information obtained from such adrill bit seismic source. Of the schemes which suggest using a sourceother than a drill bit, U.S. Pat. No. 3,909,776 to Broding et al.suggests using a fluid driven oscillator which changes emitted frequencyas a function of time (much like a "Vibroseis" source), U.S. Pat. No.3,881,168 to Farr et al. suggests using a monofrequency fluidoscillator, while U.S. Pat. No. 4,207,619 to Klaveness suggests using apulse generator located in the drill string just above the drill bit. Inaddition, U.S. Pat. No. 4,033,429 to Farr suggests providing a seismicsource in a drill string by modifying a section of a drill pipe toreduce it in diameter and provide a port, and providing a cylindricalsleeve surrounding the modified drill pipe section. The sleeve isprovided with mud ports arranged in a helical path, with the spacing ofthe ports varying from a fifteen degree spacing at one end of the helixto a one hundred twenty degree spacing at the other end of the helix. Tocreate a seismic signal, the entire drill string is lifted off of thehole bottom, and the drill pipe (and drill bit) is rotated and movesdownward. The movement of the drill pipe port past the sleeve portsgenerates a series of pressure pulses in a siren-like fashion whichchange in frequency due to the spacing of the sleeve ports. The rotationrate of the drill pipe will therefore control the specific frequencyrange of the sweep.

While the idea of utilizing a "Vibroseis" type seismic source downholeis conceptually advantageous, the techniques proposed in the art forgenerating such a source have not been feasible. In particular, thefluid driven oscillator of Broding et al. will not provide a discretepoint of signal excitation and will thereby "smear" the seismic image,and the oscillator must be inconveniently added to the drill string atextra expense. Additionally, in deviated wells, wellbore friction makessmooth rotation of the drill string impossible, causing variations insignal frequency. Finally, the drill string must be removed from theborehole each time the system is operated, forcing costly delays in thedrilling process. With respect to U.S. Pat. No. 4,033,429 to Farr, therequirement to modify and rotate the drill pipe in order to effect aseismic source is disadvantageous for several reasons. First, in Farr,the drilling process and the seismic signal generation process areexclusive of each other, such that tripping out is required to move fromdrilling to seismic signal generation and vice versa. Second, therotation of the drill pipe during seismic signal generation will causebanging on the side of the well which will generate undesirable noise.Third, because the seismic signal is being generated by a pressuredifferential across different orifices, the location where the pressureis being generated (i.e., the source signal location) will beundesirably uncertain.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a downhole seismicsource capable of generating a sweep of frequencies.

It is another object of the invention to provide a downhole seismicsource having a definitive signal location.

It is a further object of the invention to provide a downhole seismicsource which generates a signal without excessive noise.

It is an additional object of the invention to provide a downholeseismic source capable of generating a sweep of frequencies by using anMWD tool without downhole modification or tripping out.

Another object of the invention is to provide a reverse VSP systemutilizing an MWD tool as a downhole source which is capable ofgenerating a sweep of frequencies.

A further object of the invention is to provide a method of generating adownhole seismic source capable of generating a sweep of frequencieswithout significant interruption of the drilling process.

In accord with the objects of the invention, the downhole seismic sourcecomprises a frequency-controllable modulator of an MWD tool which isused to provide a large force to the drill bit which is in contact withthe formation. The frequency-controllable modulator preferably providesa substantially sinusoidal output, and is controlled to provide a"Vibroseis"-type frequency sweep. Since the bit is in contact with thebottom of the borehole, the force generated by thefrequency-controllable modulator will generate measurable axialcompressional (P-) waves. Because the bit may also be in contact withside walls of the borehole by virtue of friction or via a stabilizer,the force generated by the modulator may also provide measurabletransverse shear (S-) waves. Regardless of whether P-waves and/orS-waves are generated, the force, which depends on the peak-to-peakpressure variations developed at the modulator and the cross-sectionalarea of the modulator, will be large (e.g., as high as 3000 lbs.)

According to one preferred aspect of the invention, the MWD tool used togenerate the frequency controllable seismic signal is substantially thetool described in U.S. Pat. No. 5,375,098 to Malone et al. which ishereby incorporated by reference in its entirety herein, and is known asthe "PowerPulse" tool of Schlumberger. Accordingly, the MWD toolincludes a modulator with a rotor and stator which are arranged (asdisclosed in U.S. Pat. No. 4,847,815 to Malone which is herebyincorporated by reference in its entirety herein) to provide asinusoidal signal, a brushless DC motor having a rotating drive shaftwhich is coupled to the rotor, a position sensor coupled to the rotatingdrive shaft to provide rotational position indications, motor drivecircuitry for driving the motor, and a microprocessor coupled to theposition sensor and motor drive circuitry which causes the motor drivecircuitry to provide drive signals to the brushless DC motor based onrotational positions of the brushless DC motor and based upon desiredpositions as described by the microprocessor. As used in conjunctionwith the invention, the "PowerPulse" tool is programmed to cause therotor to generate a sinusoidal frequency sweep. The frequency sweep maybe accomplished by slowly and continuously varying the frequency overtime, or by providing a signal at a first frequency for a desirableamount of time (e.g., ten seconds), changing the frequency to a secondfrequency for a desirable amount of time, and continuing to change thefrequency until a variable frequency sweep is accomplished over adesired range of frequencies.

In accord with another preferred aspect of the invention, the frequencysweep is chosen to extend from 1 Hz to 50 Hz. According to yet anotherpreferred aspect of the invention, signal processing is accomplished byusing a kelly-mounted accelerometer or strain gauge which provides areference signal against which a cross-correlation may be done. Thisprocedure allows identification and stacking of waveforms based on theassumption of origination from a common source and reflector. A signalprocessing technique offering a substantial signal-to-noise ratio isdescribed in U.S. Pat. No. 4,926,392 by Handley.

The downhole frequency-controllable seismic source of the inventionprovides many advantages over the seismic sources of the prior art.Because the source is downhole, the resolution of the reverse VSPprocessing will be relatively good, and formations ahead of the bit maybe imaged before being reached by the MWD tool. In addition, because themodulator utilized in the invention outputs a substantially sinusoidalsignal, the modulator source can provide large amounts of energy(equivalent to a dynamite source) by remaining at desired frequenciesover periods of time (i.e., by sweeping slowly), thereby improving thesignal to noise ratio. Furthermore, because the seismic signal isapplied through the bit, the location of the source is well known.Further yet, the invention permits reverse VSP processing without addingdownhole hardware equipment, and without requiring trips out of the wellor significantly delaying the drilling process.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the reverse VSP system of theinvention.

FIG. 1a is a diagram of the MWD tool of the invention in its typicaldrilling environment.

FIG. 2 is a schematic diagram of the MWD tool of the invention whichshows how FIGS. 2a-2d relate to each other and also shows othercomponents of the MWD tool.

FIGS. 2a and 2b, and 2c and 2d are respectively partially cut-awayperspective representations, and cross sectional representations throughportions of the preferred MWD tool of the invention.

FIGS. 3a and 3b are respectively isometric and front plan views of thepreferred stator of FIG. 2d.

FIGS. 4a, 4b, and 4c are respectively isometric, front plan, and sideelevational views of the preferred rotor of FIG. 2d.

FIG. 5 is a cross sectional view of the magnetic positioner of FIG. 2c.

FIG. 6a is a block diagram of the motor drive apparatus and motorcontroller function of the invention.

FIG. 6b is a software flow diagram of the motor control software for themicroprocessor of FIGS. 2 and 6a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The system of the invention as seen in FIG. 1 includes an array orstring of surface sensors (geophones) 3, a drilling rig 4 withaccelerometers or strain gauges 5 coupled thereto, a processor 6 whichreceives and processes information from the accelerometers or straingauges and the geophones 3, and a drill string 14 which is supported bythe rig 4 and which includes an MWD tool 8, a drill bit 15, and drillbit stabilizer blades 9. As will be described in more detailhereinafter, the MWD tool 8 includes a downhole pressure pulse signalingdevice 18, and a measurement instrument package 19 (FIG. 1a). As willalso be described in more detail hereinafter, the signaling device 18comprises a frequency controllable sinusoidal modulator. According tothe preferred embodiment of the invention, when it is desired to conducta reverse VSP process, the sinusoidal modulator is controlled by aprogrammed microprocessor 91 (FIG. 2) to output a frequency sweep fromapproximately 1-50 Hz. The frequency sweep is conducted with the drillbit 15 preferably contacting the bottom of the borehole, so that the mudpulsing through the modulator applies pressure to the drill bit andhence to the formation 7. However, while the frequency sweep isconducted, the drilling is preferably stopped; i.e., no other pressureis applied to the bit other than the weight of the drill string and thepressure applied by the mud pulsing through the modulator.

It should be appreciated that the mud pulse energy is and can be appliedto the formation through different mechanisms. In the preferredembodiment, with the drill bit contacting the bottom of the borehole,the axial shaking force generated by the mud pulses of the modulatorwill generate compressional (P-) waves. The axial shaking force dependson the peak-to-peak pressure variations developed at the modulator, andthe cross-sectional area of the modulator, as the force equals thepressure times the area. With the "PowerPulse" tool of Schlumberger, a3000 lb. axial shaking force would be typical. The actual loadtransferred to the formation depends on the characteristic impedance ofthe steel drill collar versus the characteristic impedance of theformation and also the inertia and stiffness characteristics of thebottom hole assembly. The inertia effect may be reduced by using anaxial decoupling device or shock sub 20 (FIG. 1a) above the MWD tool,thereby enhancing coupling of the axial force to the formation. At thesame time, it will be appreciated that the shaking force can also becoupled to the borehole wall via the stabilizers 9 and contact frictionforces. Such a coupling will typically generate shear (S-) waves. Thus,while it is preferred that the bit be in contact with the bottom of theborehole in conducting the reverse VSP according to the invention, itwill be appreciated that the invention could also be used to generateS-waves with or without the bit being in contact with the bottom of theborehole.

As the frequency sweep is conducted, the kelly-mounted accelerometers orstrain gauges 5 record a reference signal while the geophones 3 recordsignals which are transmitted through the formation 7. VSP processing isthen conducted by the processor 6 by cross-correlating the referencesignal received by the accelerometers or strain gauges 5 to the signalsrecorded at the geophones 3. Cross-correlation uses a technique whichdefines the offset in time between the source (reference vibration) andthe signal, and allows identification and stacking of waveforms based onthe assumption of origination from a common source and reflector (commondepth point stacking). A signal processing technique offering asubstantial signal-to-noise ratio is described in U.S. Pat. No.4,926,392 to Handley. It will be appreciated that the velocity of soundin the formation in the region between the downhole source and surfacecan be accurately defined, as the first arrival at the geophones will beP-waves from the direct transit between the MWD tool source and thegeophone string (check shot survey). Later P-wave and S-wave arrivalswill occur and can be interpreted as reflections from seismicreflectors. It will also be appreciated that different signal processingtechniques can be utilized in conjunction with the preferred embodimentof the invention.

Referring to FIG. 1a, the operation of the MWD tool 8 in a typicaldrilling arrangement is illustrated schematically. Drilling mud 10 ispicked up from mud pit 11 by one or more mud pumps 12 which aretypically of the piston reciprocating type. The mud 10 is circulatedthrough mud line 13, down through the drill string 14, through the drillbit 15, and back to the surface of the formation via the annulus 16between the drill string and the wall of the well bore 29. Upon reachingthe earth's surface 31, the mud is discharged through line 17 back intothe mud pit 11 where cuttings of rock or other well debris are allowedto settle out before the mud is recirculated.

The MWD tool 8 includes a downhole pressure pulse signaling device 18for transmission of data signals derived during the drilling operationby the measurement instrument package 19. A preferred rotor and statorfor the signaling device which generates sinusoidal signals is discussedhereinafter with reference to FIGS. 3a, 3b, and 4a-4c, although asimilar device disclosed in U.S. Pat. No. 4,847,815 may also beutilized. Data signals are encoded in a desired form (also as discussedhereinafter) by appropriate electronic means in the downhole tool.Arrows 21, 22, and 23 illustrate the path taken by the pressure pulsesprovided by the downhole signaling device 18 under typical wellconditions. Pump 12 also produces pressure pulses in the mud line 13 andthese are indicated by arrows, 24, 25, 26 and 26A which also illustratethe flow of the mud through the annulus 16.

In order for the downhole pressure pulse signals to be recovered at thesurface, some means is preferably provided to remove or substantiallyeliminate the portion of the mud pressure signal due to the mud pumps.Subsystem 30, including pressure transducer 32, mud pump piston positionsensors 34, and computer or processor 36, comprises one possible suchmeans and is disclosed in detail in U.S. Pat. No. 5,146,433 which ishereby incorporated by reference herein.

Some of the more pertinent details of the MWD tool 8 are seen withreference to FIGS. 2 and 2a-2d. In FIGS. 2a-2d, the tool 8 is seeninside and supported by a drill collar 52. Thus, as seen in FIG. 2a, thetool 8 is provided with a shoulder 54 which supports the tool in thedrill collar 52. Also seen in FIG. 2a are a local tool bus extender 56which provides power and a data link to other sensors.

As seen in FIG. 2b, a turbine 58 is provided. The turbine includes aturbine rotor 60, a turbine stator 62, and a turbine shaft 64. Theturbine 58 is driven by the mud circulating through the borehole and theMWD tool. As the mud pushes by the turbine 58, the turbine shaft 64rotates. The turbine shaft 64 is coupled to an alternator 70 which usesthe rotating shaft to generate an electric signal which is rectified fordriving (powering) the brushless DC servo motor 100 (see FIG. 2c) andallowing the motor 100 to operate.

Turning to FIG. 2, as seen in schematic form, and located between thealternator 70 (of FIG. 2b) and the motor 100 (of FIG. 2c), are apressure bulkhead 84, sensors 19 (inclinometers, etc.), an electronicspackage 90 including a microprocessor 91 (details of which will bediscussed hereinafter with reference to FIGS. 6a, and 6b), and apressure compensator 92. The pressure bulkhead 84 and compensator 92keep the electronics package 90 and sensors 19 at or near atmosphericpressure so that they may function properly.

The brushless DC servo motor 100 which drives the rotor 160 (see FIG.2d) of the MWD tool 8 is seen in FIG. 2c. In the preferred embodiment,the motor is a motor available from MOOG of East Aurora, N.Y. under part#303F052, and includes a motor shaft/motor 102, magnets 106, and a motorstator 108. On the tail end 112 of shaft 102 of the motor is located aposition sensor 110 sold under part #JSSBH-15-C-1/P137 by the CliftonPrecision subsidiary of Litton Systems, Inc., Clifton Heights, Pa. Thefunction of the position sensor 110 is to determine exactly how far theshaft 102 has rotated. Preferably, the position sensor 110 resolves asingle rotation of the shaft into four thousand ninety-six counts(twelve bits).

The driving end 114 of shaft 102 is coupled to a gear train 120 whichreduces the rotation by a factor of eight; although it should beappreciated that other gear reductions or no gear reduction could beutilized depending upon the capabilities of the motor, and upon thefrequencies which need to be generated. In the gear train 120, however,the first gears 122a and 122b of the gear train effect a 2:1 reductionin rotation speed. Located on the shaft 124 coupled to gear 122b is amagnetic positioner 130, discussed in detail hereinafter with referenceto FIG. 5. The function of the magnetic positioner 130 is to prevent themodulator 18 (seen in FIG. 2d) from getting stuck in a closed position,and thereby preventing mud from circulating down through the MWD tooland driving the turbine 58.

As seen in FIG. 2c, the gear train 120 also includes gears 132a and 132bwhich effect a further 4:1 reduction in rotation speed of the shaft.Thus, the rotor 160 seen in FIG. 2d, rotates one time for every eightrevolutions of the motor 100. Because the rotor 160 (as discussed inmore detail with reference to FIGS. 3a, 3b, and 4a-4c) has four lobes,one full rotation of the rotor 160 relative to the stator 150 of FIG. 2dgenerates a signal approximating four sinusoids. With the eight to onereduction, two revolutions of the motor 100 are required to generate asingle sinusoid from the signaling device 18 which includes the rotor160 and stator 150 together. Again, as suggested above, depending uponthe capabilities of the motor, different gear reduction ratios may beimplemented in order to generate the desired frequency sweep.

FIGS. 3a and 3b are respectively isometric and front plan views of thepreferred stator 150 of the invention. The stator 150 and the rotor 160(shown in FIGS. 4a, 4b, and 4c) generally comply with the teachings ofU.S. Pat. No. 4,847,815 and generate sinusoidal waves. In particular,the stator 150 is seen with four lobes 171a, 171b, 171c, and 171d. Eachlobe has a first side 152 a second side 154 and an outer edge 156. Asseen in FIG. 3b, the first side 152 is radial from the origin O of thestator. However, instead of the second side 154 of the lobe beingparallel with the first side 152 (as taught in the preferred embodimentof U.S. Pat. No. 4,847,815), as shown in FIG. 3b, they are at an angleof approximately thirteen degrees relative to each other. Also, as shownin FIG. 3b, but seen better in FIG. 3a, the lobes 171 of the stator areundercut at an angle as seen at 158.

Turning to FIGS. 4a, 4b, and 4c, isometric, front plan, and sideelevational views of the preferred rotor 160 are seen. The rotor 160, asdiscussed above with reference to FIGS. 2a-2d is coupled to a driveshaft which rotates the rotor 160 relative to the stator 150, therebygenerating a signal. As with the stator 150, the rotor 160 has fourlobes 172a, 172b, 172c, and 172d. Each lobe has a first side 162, asecond side 164, and an outer edge 166. As seen in FIG. 4b, the firstside 162 is radial from the origin A of the rotor. The second side 166of the lobe is at an angle of approximately thirteen degrees relative tothe first side 164. With the provided geometry of the stator 150 inconjunction with the similar geometry of the rotor 160, when the rotoris at a steady speed, the orifice between the rotor and the statorvaries in time substantially with the inverse of the square root of alinear function of a sine wave (as discussed in detail in U.S. Pat. No.4,847,815). The resulting signal is therefore generally sinusoidal innature.

FIG. 5 is a cross sectional view of the magnetic positioner 130 of FIG.2c. The magnetic positioner is comprised of four sets of magnets 130aS,130aN, 130bS and 130bN. Two of the four sets of magnets 130aS and 130aNare coupled to the drive shaft 124 and rotate therewith. Inner magnets130aS, as shown are "south" polarity magnets and extend one hundredeighty degrees around the drive shaft 124, while magnets 130aN, are"north" polarity magnets which extend the other one hundred eightydegrees around the drive shift 124. Axially displaced from andsurrounding magnets 130aS and 130aN, and fixed to the housing 130c ofthe magnetic positioner are outer magnets 130bS and 130bN. Outer magnets130bS (south polarity magnets) extend one hundred eighty degrees aroundmagnets 130aS and 130aN, and outer magnets 130bN (north polarity)extending the other one hundred and eighty degrees around the innermagnets.

With the magnetic positioner 130 as provided, the rotor 150 is preventedfrom getting stuck in a closed position relative to the stator 150, andthereby preventing mud from circulating down through the MWD tool anddriving the turbine 58. In particular, during jamming or during apower-down state, the magnets of magnetic positioner 130 will try toalign themselves as shown in FIG. 5, with the south polarity innermagnets 130aS opposite the north polarity outer magnets 130bN, and thenorth polarity inner magnets 130aN opposite the south polarity outermagnets 130bS. The alignment of the magnets, causes the drive shaft 124to rotate from whatever position it was in, to the position of FIG. 5.The rotation of the drive shaft in turn causes the rotor 160 to rotate.By placing the rotor 160 on its drive shaft in an "open" orientationrelative to the stator 150 when the magnets are aligned as shown in FIG.5, whenever the magnets return to the position of FIG. 5, the rotor 160will be open relative to the stator 150. It will be appreciated thatbecause of the 4:1 step down in gears (reduction), a one hundred eightydegree rotation of the drive shaft 124 of the magnetic positioner, willonly effect a forty-five degree rotation of the drive shaft of the rotor160. However, because the rotor 160 has four lobes, a forty-five degreerotation causes a rotor in a fully closed state to rotate into a fullyopen state.

As aforementioned, the turning of the rotor 160 of the signaling device18 generates a sinusoidal signal. In order to generate the frequencysweep signal (as well as signals which can be used to transmit downholedata to the surface equipment for detection, processing, and decoding),the rotation of the rotor 160 is controlled by the motor 100 which inturn is controlled by the microprocessor 91. In the preferredembodiment, the microprocessor 91 is programmed in order to permit themodulator to generate any frequency up to 50 Hz. Themeasurement-while-drilling techniques for transmitting data through themud, which are not directly pertinent to this invention, are disclosedin detail in previously incorporated U.S. Pat. No. 5,375,098.

In order to change the phase and/or frequency of the signal, therotation of the rotor 160 is controlled by the motor 100. In turn, therate at which the motor turns is controlled by a drive controller 191(seen in FIG. 6a) under instructions of the microprocessor 91. Anoverview of this system is seen in FIG. 6a. As seen in block diagramform in FIG. 6a, and previously discussed with reference to FIG. 2c,coupled to the motor 100 (and typically on the motor shaft 102) is theposition sensor or resolver 110. The shaft 102 is optionally steppeddown by a 2:1 geartrain 120 to which the magnetic position 130 iscoupled. Another geartrain 132a/132b is optionally provided to effect anadditional 4:1 step down in rotation, and the four-lobed signalingdevice or modulator 18 is coupled thereto. As seen in FIG. 6a, theoutput of the position sensor 110 is provided to the microprocessor 91.The microprocessor, in turn, provides a duty cycle signal to the motorcontroller 191 which effectively pulse width modulates a DC power bus192 to the motor 100, thereby controlling the speed of the motor. Thus,a feedback arrangement is set up, whereby if the motor moves the rotortoo much (as sensed by the position sensor 110), the duty cycle isreduced by the microprocessor 91 and the drive signal of the controller191 is reduced; while if the motor does not move the rotor enough, theduty cycle is increased, and the drive signal of the controller 191 tothe motor 100 is increased.

Controlling the modulator over varying mud flow rates and mud densitiesrequires the motor software to perform several tasks in order to ensurethe generation of a readable signal. In particular, the voltage producedby the alternator is roughly proportional to the flow rate, while theload on the modulator increases with increasing flow rate and mudweight. In order to control the modulator, an adaptive PD controlalgorithm is used for the motor (with a proportional--P term, and aderivative--D term), with gains being constantly adjusted to compensatefor the varying bus voltages and loads encountered. It will beappreciated that while an adaptive PD control algorithm is preferred,other control algorithms known in the art can also be used.

In FIG. 6b, a high level software flow diagram is seen of the motorcontrol software for the microprocessor 91 of FIGS. 2 and 6a whichcauses the frequency sweep to be accomplished. It will be appreciatedthat additional software is utilized by the microprocessor 91 to effecttransmission of data via the mud as is described in detail in previouslyincorporated U.S. Pat. No. 5,375,098. Thus, as seen in FIG. 6b, afterthe mud flow has started at 202 and the power to the MWD tool has beensupplied, the CPU resets at 204. At 206, a decision is made as towhether the MWD tool is to be used to convey information (MWD transmitmode) or to generate a seismic signal (seismic mode). If the MWD tool isto be used to convey information via the mud, the program continues atstep 220. However, if the MWD tool is to be used to provide a seismicsignal, a determination is made at 208 as to whether weight is beingapplied to the bit. If not, at 209, a message is sent to the surface(via mud pulse telemetry) to apply weight to the bit, and the programcontinues at step 206. If weight is being applied to the bit, at 210,the modulator 18 is controlled to provide a fixed frequency (e.g., 1Hz). At step 212, the frequency is increased by a desired amount. Atstep 214, if the frequency has reached a desired maximum (e.g., 50 Hz),the program continues at 220 by moving into the MWD transmit mode. Ifthe maximum desired frequency has not been reached, the programcontinues at step 212 by increasing the frequency by a desired amount.It will be appreciated by those skilled in the art that the amount thatthe frequency of the modulator is increased at step 212 depends on thefrequency at which steps 212 and 214 are carried out. In other words, ifthe frequency is checked every millisecond, it may be desirable toincrease the frequency of the modulator by an amount on the order of amillihertz. However, if the frequency is checked only every half asecond, it may be desirable to increase the frequency of the modulatorby an amount on the order of 0.1 Hz.

It should be appreciated that instead of running a frequency sweep offrom 1 Hz to 50 Hz, different frequency sweeps could be utilized. Forexample, using the Schlumberger "PowerPulse" tool with changes only tothe software, the frequency sweep might more typically range from 0 to30 Hz. However, the "Vibroseis"-type processing would remainsubstantially the same, with account being made for the differentfrequency range.

With the software, apparatus, and system of the invention as described,it will be appreciated that the method of the invention generallycomprises locating the drill string with the MWD tool as describeddownhole, and with the drill bit preferably engaging the bottom of theborehole, generating a frequency sweep by causing the rotor of the MWDtool modulator to ramp its speed over a period time. The generallysinusoidal output of the modulator is preferably caused to sweep over arange from 1 to 50 Hz. At the surface, the P-wave and/or S-wave signalsgenerated downhole are detected by an array of geophones, while thekelly-mounted accelerometer or strain gauge obtains a reference signal.The reference signal and the signals obtained by the geophones are thenutilized in manners known in the art to provide seismic profilinginformation (e.g., imaging) regarding the formation.

There have been described and illustrated herein apparatus, systems andmethods for seismic profiling which utilize the modulator of an MWD toolas a downhole seismic source. While particular embodiments of theinvention have been described, it is not intended that the invention belimited thereto, as it is intended that the invention be as broad inscope as the art will allow and that the specification be read likewise.Thus, while the invention has been described with reference to apreferred MWD tool (the "PowerPulse" tool of Schlumberger), it will beappreciated that other MWD tools which utilize a modulator to generatesubstantially sine wave outputs could be used. Likewise, while apreferred high level software flow diagram was provided for the downholemicroprocessor in order to cause the modulator to generate a frequencysweep, it will be appreciated that other software, firmware or hardwarecould be utilized. Also, it will be appreciated that while preferred VSPprocessing techniques were disclosed which utilize the reference signaland the signals obtained by geophones at the formation surface, otherprocessing techniques could be utilized. Likewise, while the inventionwas described with reference to an uphole signal causing themicroprocessor downhole to start the software routine for causing themodulator to generate a frequency sweep, it can be appreciated that thefrequency sweep could be generated automatically downhole based ondesired parameters; e.g., after drilling a certain distance, or eachtime drilling must be stopped, or each time a change in formationdensity is observed, etc. It will therefore be appreciated by thoseskilled in the art that yet other modifications could be made to theprovided invention without deviating from its spirit and scope as soclaimed.

We claim:
 1. A method for generating a seismic signal in a formationtraversed by a borehole having borehole fluid therein to obtaininformation regarding said formation, and utilizing a borehole toolhaving formation contact means for coupling the borehole tool to theformation, a motor with a rotating drive shaft, a rotor coupled to thedrive shaft, a stator which is stationary relative to the rotor, withthe rotor and stator arranged to provide substantially sinusoidalsignals in the borehold fluid as the rotor turns relative to the stator,a microprocessor means coupled to the motor for causing the motor toturn the rotor, said method comprising:a) coupling said borehole tool tothe formation utilizing the formation contact means; b) controlling saidmotor with said microprocessor means so as to turn said rotor such thatsaid substantially sinusoidal signals change in frequency in a frequencysweep over a predetermined frequency range; c) imparting a force againstsaid formation via said contact means to produce a seismic signal; andd) receiving seismic waveform information relating to said seismicsignal with at least one geophone located on or near a surface of saidformation.
 2. A method according to claim 1, wherein said formationcontact means is a drill bit or at least one stabilizer blade, step (a)comprising:causing the drill bit to contact the bottom of the boreholeor causing at least one stabilizer blade to contact a side of theborehole.